Gas up: A cyclometalated iridium complex is found to catalyze the dehydrogenation of various benzofused N‐heterocycles, thus releasing H2. Driven by as low as 0.1 mol % catalyst, the reaction affords quinolines, indoles, quinoxalines, isoquinolines, and β‐carbolines in high yields.

Noncovalent interactions, such as hydrogen bonding, electrostatic, p-p, CH-p, and hydrophobic forces, play an essential role in the action of natures catalysts, enzymes. In the last decade these interactions have been successfully exploited in organocatalysis with small organic molecules. [1] In contrast, such interactions have rarely been studied in the wellestablished area of organometallic catalysis, [2] where electronic interactions through covalent bonding and steric effects imposed by bound ligands dictate the activity and selectivity of a metal catalyst. An interesting question is: What happens when an organocatalyst meets an organometallic catalyst? This unification has already created an exciting new space for both fields: cooperative catalysis, where reactants are activated simultaneously by both types of catalyst, thereby enabling reactivity and selectivity patterns inaccessible within each field alone. [3] However, the mechanisms by which the two catalysts cooperatively effect the catalysis remain to be delineated. We recently found that combining an achiral iridium catalyst with a chiral phosphoric acid allows for highly enantioselective hydrogenation of imines (Scheme 1). [4] To gain insight into the mechanism of this metal-organo cooperative catalysis, we studied the catalytic system with a range of techniques, including high pressure 2D-NMR spectroscopy, diffusion measurements, and NOEconstrained computation. Herein we report our findings.To evaluate the mechanism, a simplified achiral complex C was used, which leads to [C + ][A À ] upon mixing, in situ or ex situ, with the chiral phosphoric acid HA through protonation at the amido nitrogen (Scheme 1). In the asymmetric hydrogenation of the model ketimine 1 a, [C + ][A À ] afforded 95 % ee and full conversion. On the basis of related studies, [5] the hydrogenation can be broadly explained by the catalytic cycle shown in Scheme 1, that is, [C + ][A À ] activates H 2 to give the hydride D and protonated 1 a, which forms an ion pair with the phosphate affording [1 a + ][A À ]; [6,7] hydride transfer furnishes the amine product 2 a while regenerating [C + ][A À ]. Questions pertinent to possible iridium-phosphate cooperation then arise: 1) How does the chiral phosphoric acid induce asymmetry in the hydrogenation? and 2) Does the enantioselectivity result from D being formed enantioselectively from [C + ][A À ], from the phosphate salt [1 a + ][A À ], or from interactions involving all three components?We looked first at how the formation of hydride D and its transfer into the substrate are influenced by the chiral acid HA. The studies were carried out in CH 2 Cl 2 or CD 2 Cl 2 owing to the low solubility of the various metal complexes in toluene. The catalytic hydrogenation is feasible in both solvents, giving a 95 % ee in toluene and 85 % ee in CH 2 Cl 2 in the case of hydrogenation of 1 a with C and HA under the conditions given in Scheme 1. The solution NMR studies show that the ionic complex [C + ][A À ] is formed instantly on protonation of C (...

Noncovalent interactions, such as hydrogen bonding, electrostatic, p-p, CH-p, and hydrophobic forces, play an essential role in the action of natures catalysts, enzymes. In the last decade these interactions have been successfully exploited in organocatalysis with small organic molecules. [1] In contrast, such interactions have rarely been studied in the wellestablished area of organometallic catalysis, [2] where electronic interactions through covalent bonding and steric effects imposed by bound ligands dictate the activity and selectivity of a metal catalyst. An interesting question is: What happens when an organocatalyst meets an organometallic catalyst? This unification has already created an exciting new space for both fields: cooperative catalysis, where reactants are activated simultaneously by both types of catalyst, thereby enabling reactivity and selectivity patterns inaccessible within each field alone. [3] However, the mechanisms by which the two catalysts cooperatively effect the catalysis remain to be delineated. We recently found that combining an achiral iridium catalyst with a chiral phosphoric acid allows for highly enantioselective hydrogenation of imines (Scheme 1). [4] To gain insight into the mechanism of this metal-organo cooperative catalysis, we studied the catalytic system with a range of techniques, including high pressure 2D-NMR spectroscopy, diffusion measurements, and NOEconstrained computation. Herein we report our findings.To evaluate the mechanism, a simplified achiral complex C was used, which leads to [C + ][A À ] upon mixing, in situ or ex situ, with the chiral phosphoric acid HA through protonation at the amido nitrogen (Scheme 1). In the asymmetric hydrogenation of the model ketimine 1 a, [C + ][A À ] afforded 95 % ee and full conversion. On the basis of related studies, [5] the hydrogenation can be broadly explained by the catalytic cycle shown in Scheme 1, that is, [C + ][A À ] activates H 2 to give the hydride D and protonated 1 a, which forms an ion pair with the phosphate affording [1 a + ][A À ]; [6,7] hydride transfer furnishes the amine product 2 a while regenerating [C + ][A À ]. Questions pertinent to possible iridium-phosphate cooperation then arise: 1) How does the chiral phosphoric acid induce asymmetry in the hydrogenation? and 2) Does the enantioselectivity result from D being formed enantioselectively from [C + ][A À ], from the phosphate salt [1 a + ][A À ], or from interactions involving all three components?We looked first at how the formation of hydride D and its transfer into the substrate are influenced by the chiral acid HA. The studies were carried out in CH 2 Cl 2 or CD 2 Cl 2 owing to the low solubility of the various metal complexes in toluene. The catalytic hydrogenation is feasible in both solvents, giving a 95 % ee in toluene and 85 % ee in CH 2 Cl 2 in the case of hydrogenation of 1 a with C and HA under the conditions given in Scheme 1. The solution NMR studies show that the ionic complex [C + ][A À ] is formed instantly on protonation of C (0....

Asymmetric hydrogenation of imines leads directly to chiral amines, one of the most important structural units in chemical products, from pharmaceuticals to materials. However, highly effective catalysts are rare. This article reveals that combining an achiral pentamethylcyclopentadienyl (Cp*)-iridium complex with a chiral phosphoric acid affords a catalyst that allows for highly enantioselective hydrogenation of imines derived from aryl ketones, as well as those derived from aliphatic ones, with ee values varying from 81 to 98 %. A range of achiral iridium complexes containing diamine ligands were examined, for which the ligands were shown to have a profound effect on the reaction rate, enantioselectivity and catalyst deactivation. The chiral phosphoric acid is no less important, inducing enantioselection in the hydrogenation. The induction occurs, however, at the expense of the reaction rate.

Heterocyclen von H2 befreit: Ein cyclometallierter Iridiumkomplex katalysiert die Dehydrierung verschiedener benzanellierter N‐Heterocyclen unter Freisetzung von H2. Mit nur 0.1 Mol‐% des Katalysators liefert die Reaktion Chinoline, Indole, Chinoxaline, Isochinoline und β‐Carboline in hohen Ausbeuten.

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Cooperative Catalysis: Combining an Achiral Metal Catalyst with a Chiral Broensted Acid Enables Highly Enantioselective Hydrogenation of Imines. -The catalytic system of title can be employed on imines derived from aromatic as well as aliphatic ketones. This method affords a broad range of chiral amines in high enantioselectivities. -(TANG, W.; JOHNSTON, S.; LI, C.; IGGO, J. A.; BACSA, J.; XIAO*, J.; Chem. -Eur.

Catalyst. -Catalytic dehydrogenation of a wide variety of benzo fused N-heterocycles is achieved in the presence of a cyclometalated iridium complex as the catalyst in the absence of any oxidant or acceptor for hydrogen. -(WU, J.; TALWAR, D.; JOHNSTON, S.; YAN, M.; XIAO*, J.; Angew. Chem., Int. Ed. 52 (2013) 27, 6983-6987, http://dx.